HelloWorld幾乎是每一個程式設計師學習過程中必會遇到的，所謂溫故而知新，今天我們來深入瞭解一下Android中的helloworld程式。

   首先，在eclipse中新建一個Android專案：

然後在MainActivity的onCreate方法中打一個斷點

然後除錯，進入除錯介面，在除錯介面中可以看到這樣的東西，這裡就是我們的程式執行時的方法棧

將這裡的方法劃分為幾個部分

如圖中所示：

Android程式啟動流程大約可以分為6個部分：

1. zygote孵化
2. 執行ActivityThread的main方法
3. 執行main方法，輪詢
4. 輪詢到訊息，派發訊息
5. 例項化並啟動Activity
6. 呼叫Activity的onCreate方法

public static void main(String[] args) {
		 
		...省略部分我看不懂的程式碼（hah）
        
		Looper.prepareMainLooper();
		
		...
        
		Looper.loop();//開始輪詢

        //一般來說程式不會執行到這一行，為什麼不會執行到這裡呢，我們去看看Looper.loop();
		throw new RuntimeException("Main thread loop unexpectedly exited");
	}
 

public static void loop() {
	        final Looper me = myLooper();//獲取當前執行緒的Looper，main方法中Looper.prepareMainLooper();已經初始化了當前執行緒的Looper
	        if (me == null) {
	        	//丟擲異常說明沒有呼叫Looper.prepare()來初始化Looper
	            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
	        }
	        final MessageQueue queue = me.mQueue;//獲取訊息佇列

	        ...

	        for (;;) {//這裡就是一個死迴圈，這裡我不明白為什麼不用while，兩者有什麼區別，知道的童靴請告訴我一聲哈
	            Message msg = queue.next(); // might block
	            if (msg == null) {
	                // No message indicates that the message queue is quitting.
	                return;//只有訊息為null的情況下才會退出迴圈
	            }

	           ...刪繁就簡，抓住問題的核心

	           msg.target.dispatchMessage(msg);//派發訊息的處理事件
	           ...
	            
	           msg.recycle();
	        }
	    }

private class H extends Handler {
        public static final int LAUNCH_ACTIVITY         = 100;
        ...
        public void handleMessage(Message msg) {
        	...
        	switch (msg.what) {
                case LAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;

                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null);//呼叫ActivityThread的handleLaunchActivity()方法
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                ...
        }
        ...
	}

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        ...
        //主要就是這一句
        Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
          ...
        } else {
            // If there was an error, for any reason, tell the activity
            // manager to stop us.
            try {
                ActivityManagerNative.getDefault()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null);
            } catch (RemoteException ex) {
                // Ignore
            }
        }
    }

private Activity performLaunchActivity(ActivityClientRecord r,
			Intent customIntent) {
		// System.out.println("##### [" + System.currentTimeMillis() +
		// "] ActivityThread.performLaunchActivity(" + r + ")");

		ActivityInfo aInfo = r.activityInfo;
		if (r.packageInfo == null) {
			r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
					Context.CONTEXT_INCLUDE_CODE);
		}

		ComponentName component = r.intent.getComponent();
		if (component == null) {
			component = r.intent.resolveActivity(mInitialApplication
					.getPackageManager());
			r.intent.setComponent(component);
		}

		if (r.activityInfo.targetActivity != null) {
			component = new ComponentName(r.activityInfo.packageName,
					r.activityInfo.targetActivity);
		}
		// 準備構造Activity
		Activity activity = null;
		try {
			java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
			// 這裡就是通過反射構造Activity
			activity = mInstrumentation.newActivity(cl,
					component.getClassName(), r.intent);
			StrictMode.incrementExpectedActivityCount(activity.getClass());
			r.intent.setExtrasClassLoader(cl);
			if (r.state != null) {
				r.state.setClassLoader(cl);
			}
		} catch (Exception e) {
			if (!mInstrumentation.onException(activity, e)) {
				throw new RuntimeException("Unable to instantiate activity "
						+ component + ": " + e.toString(), e);
			}
		}
		try {
			Application app = r.packageInfo.makeApplication(false,
					mInstrumentation);
			...
			if (activity != null) {
				Context appContext = createBaseContextForActivity(r, activity);
				...
				//呼叫Activity的attach方法
				activity.attach(appContext, this, getInstrumentation(),
						r.token, r.ident, app, r.intent, r.activityInfo, title,
						r.parent, r.embeddedID,
						r.lastNonConfigurationInstances, config);

				...
				//呼叫Activity的OnCreate方法
				activity.mCalled = false;
				mInstrumentation.callActivityOnCreate(activity, r.state);
				...
			}
			r.paused = true;

			mActivities.put(r.token, r);

		} catch (SuperNotCalledException e) {
			throw e;

		} catch (Exception e) {
			if (!mInstrumentation.onException(activity, e)) {
				throw new RuntimeException("Unable to start activity "
						+ component + ": " + e.toString(), e);
			}
		}
		return activity;
	}